Cathodic protection is a known technique for controlling corrosion in a buried or submerged metal structure. Cathodic protection is utilized to protect pipelines, missile sites, or other buried or submerged structures from decay due to corrosion. However, cathodic protection systems must be monitored in order to ensure satisfactory operation and proper corrosion control. One known approach for monitoring the performance of a cathodic protection system involves measuring the electric potential between the buried or submerged structure and a reference electrode using a DC voltmeter. This procedure is sometimes referred to as measuring the cathodic protection potential of the system. Such potential measurement is indicative of the chemical activity at the interface between the metal structure and the aggressive medium, i.e., soil, water, etc. As a result, the potential measurement provides, at least in the ideal case, an indication as to the effectiveness of the cathodic protection system.
However, oftentimes the cathodic protection potential as it is measured between the structure and the reference electrode will be erroneous as a result of what is known as IR drop. IR drop is known to affect the measured potential between the structure and the reference electrode, and the resulting measured potential often misrepresents the effectiveness of the cathodic protection system. Additional detail regarding the cause of IR drop and how IR drop can complicate the measuring of cathodic protection potentials is provided in U.S. Pat. No. 4,080,565 and in the following technical papers: Cathodic Protection Potentials Without IR Drop: A New Instrument System Solves the Problem, Andrew L. Smart and Karl W. Nicholas, as presented at the National Association of Corrosion Engineers CORROSION-88 (March 1988) and which discusses measuring cathodic protection potentials using a pulse generator which interrupts the DC current to the structure, and IR Drop in Cathodic Protection Measurements, James B. Bushman, (Ohio: CORRPRO Companies, Inc., 1984). The entire disclosures of the above patent and technical papers are incorporated herein by reference.
Unfortunately, previous attempts at measuring cathodic protection potentials substantially IR drop free have resulted in erroneous measurements and/or a misrepresentation of the effectiveness of the cathodic protection system. One approach has been to measure the polarized potential between the reference electrode and an auxiliary electrode or coupon adjacent to the metal structure. However, a problem associated with measuring the polarized potential of the coupon is ensuring that the measurement is taken at an appropriate time when the polarized potential of the coupon reaches a relative steady state upon being decoupled from the metal structure.
As will be explained more fully below, it has been discovered that when the coupon is decoupled from the metal structure, the potential of the coupon undergoes rapid fluctuations, ringing, or the like, which can affect the validity of the measurement. For example, there may be inductive and/or capacitive spikes which occur in the polarized potential of the coupon following the decoupling of the coupon from the metal structure. The presence of such inductive or capacitive spikes or the like will appear in the polarized potential measurement, and, as a result, the measured potential will misrepresent the effectiveness of the cathodic protection system, as is explained more fully below. Possible factors which can contribute to inductive and/or capacitive spikes in the polarized potential of the coupon include the type, quality, and impedance of the insulation or coating on the metal structure, if any, and the impedance of the soil or other aggressive medium
Furthermore, research has shown that a significant depolarization or decay can occur in the polarized potential of the coupon after decoupling, even after only approximately 100 msec or less. The charge on the coupon naturally tends to depolarize once the current is interrupted, and the measured polarized potential typically will misrepresent the effectiveness of the protection system if significant depolarization occurs. Depending on the factors which make up the overall RC time constant of the measurement system, such as the insulation of the coupon, the impedance of the soil/aggressive medium, etc., the coupon may undergo depolarization quite rapidly. In such cases, it is difficult, and often impossible, to obtain a legitimate reading using a voltmeter. The rapidly changing digital voltmeter display is difficult if not impossible to read and is subject to misinterpretation.
In view of the above-described shortcomings of existing IR drop free measurement systems for monitoring a cathodic protection system, there is a strong need in the art for a system which can provide accurate and repeatable polarized potential measurements substantially IR drop free. There is a strong need for a cathodic protection monitoring system which measures the polarized potential at predetermined and adjustable times so as to avoid the problems associated with voltage spikes or depolarization in the polarized potential of the coupon. In addition, there is a need for a system which is noise-free or virtually noise-free and which produces accurate measurements regardless of the specific test site. Furthermore, there is a strong need in the art for a cathodic protection monitoring system which is economical and portable. Such a cathodic protection monitoring system can be carried from site to site and used on different buried or submerged structures.
The use of a coupon monitoring system enables one easily to examine the coupon at any future time to determine that the criteria one is using in preventing corrosion is effective. This is far less costly than exhuming the buried structure to determine the extent of and where such corrosion is occurring.